You Do Need power plug adapter while traveling from Saint Helena to Thailand.

Here is Why You Need An Adapter

The outlet and plug type used in Thailand are A, B, C and O and in Saint Helena is G. This is how they look like.

Saint Helena

In Saint Helena, outlets of type G are in use.

Type G

Thailand

In Thailand, outlets of type A, B, C and O are in use.

Type A

Type B

Type C

Type O